The ability of polymers to act as electrical insulators is the basis for their widespread use in the electrical and electronic fields. However, material designers have sought to combine the fabrication versatility of polymers with many of the electrical properties of metals. There are instances when an increased conductivity or relative permitivity of the polymer is warranted, such as in applications which require antistatic materials, low-temperature heaters, electromagnetic radiation shielding and electric field grading. A few select polymers, such as polyacetylene, polyaniline, polypyrrole and others, can be induced to exhibit intrinsic electronic conductivity through doping, though these systems tend to be cost prohibitive and difficult to fabricate into articles.
Conductivity may be imparted to a polymer through the creation of conducting polymer composite materials. Electronic conducting polymer composite materials require a random distribution of a conducting filler to be dispersed throughout an insulating polymer which results in a infinite network capable of supporting electron flow. A material is considered conducting when its volume resistivity drops below about 10.sup.8 .OMEGA..multidot.cm to about 10.sup.6 .OMEGA..multidot.cm. When conducting filler in a polymer is distributed at a volume level sufficient to support electron flow in a polymer/conducting filler blend, a continuous conducting network exists in the polymer.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold which is defined as the lowest concentration of conducting particles at which continuous conducting chains are formed, e.g., when a continuous conducting network is generated, is easily determined from the experimentally determined dependence of conductivity of the conducting polymer composite material on the filler concentration. For a general discussion on percolation theory, see the October 1975 Vol. 45, No. 4, Review of Modem Physics article, entitled, Percolation and Conduction, the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the percolation threshold with regard to the conducting filler material. See for example, Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conducting and Insulating Materials, 1993 Journal of Materials Science, Vol. 28; Resistivity of Filled Electrically Conducting Crosslinked Polyethylene, 1984 Journal of Applied Polymer Science, Vol. 29; and Electron Transport Processes in Conductor-Filled Polymers,1983 Polymer Engineering and Science Vol. 23, No. 1; the contents of each of which are herein incorporated by reference.
Conducting composite polymer materials may be created through a "percolation-within-percolation" approach. For example, two immiscible polymers can be identified; the .alpha.-polymer which is selectively filled with a conducting filler, and the .beta.-polymer which is to be filled with the .alpha.-polymer conducting filler blend. Denoting the critical weight fraction, or percolation threshold, of the conducting filler required to insure conductivity in the .alpha.-polymer as p.sub..alpha. and the critical weight fraction or percolation threshold of the .alpha.-phase required to insure conductivity of the .alpha. polymer/conducting filler blend in .beta. as p.sub..beta., the critical weight fraction or percolation threshold of the conducting filler in the total ternary blend can be calculated based on the following mathematical equation: EQU p.sub.c =p.sub..alpha. p.sub.62 (1)
Extending the approach to higher levels of percolation, the critical weight fraction or threshold of the conducting filler in the blend can be calculated based on the following mathematical equation. EQU p.sub.c =p.sub..alpha. p.sub.62 . . . p.sub.n-1 p.sub.n (2)
where p.sub.n is the percolation threshold of co-continuity of the (n-1)-polymer blend in the n-polymer, and allows, at least theoretically, for the feasibility to obtain a conducting composite with as low a level of conducting filler as desired via multiple percolation. This "multiple percolation" approach to forming conducting polymer composites has been reported in the scientific literature, see for example Multiple Percolation in Conducting Polymer Blends, 1993 Macromolecules Vol. 26, the contents of which is herein incorporated by reference.
Applications of the heretofor described alternatives for reduction of conducting filler content in conducting polymer composite materials have been reported for polyethylene/polystyrene immiscible blends and for polypropylene/polyamide immiscible blends, both employing carbon black as the conducting filler. See for example, Design of Electrical Conducting Composites: Key Role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends, 1995 Macromolecules, Vol. 28 No. 5 and Conducting Polymer Blends with Low Carbon Black Loading: Polypropylene/Polyamide, 1996 Polymer Engineering and Science, Vol. 36, No. 10, the contents of both of which are herein incorporated by reference.